1. Field of the Invention
The present invention relates to a method for manufacturing a bar-shaped preform, which method comprises the following steps:
i) supplying one or more reactive gases, which may or may not be doped, to the interior of a substrate tube,
ii) carrying out a deposition reaction of gases from step i) in said substrate tube so as to obtain one or more glass layers on the interior of the substrate tube, and
iii) contracting the substrate tube from step ii) while heating said tube so as to obtain the massive, bar-shaped preform.
The present invention further relates to a method for manufacturing optical fibres, using such a bar-shaped preform.
2. Discussion of the Background
Such a method is known per se from U.S. Pat. No. RE 30.635. According to said US patent, a PCVD (Plasma Coating Vapor Deposition) process is used for manufacturing internally clad glass tubes from which optical fibres are drawn, which tubes consists of a core and a jacket of glass, which core and which jacket have different refractive index values. In order to obtain an optical fibre having a low attenuation, due to the presence of OH groups, at a wavelength of 1385 nm, a hydrogen-free fluor compound is generally supplied to the hollow substrate tube. The addition of such a fluor compound prevents the incorporation of OH-groups during the PCVD process. The incorporation of fluor in the glass structure causes the refractive index value of the glass to decrease, however. Said decrease is subsequently compensated by adding to the reactive gas mixture a component which has properties that cause the refractive index to increase, for example GeO2. One drawback of such a method is the fact that it leads to an increase of the total amount of dopes in the glass structure, resulting in an undesirable attenuation increase caused by the so-called Rayleigh scattering.